代碼體現(xiàn)loadClass方法
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
if (parent != null) {
c = parent.loadClass(name, false);
} else {
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
類的加載流程
-
findLoadedClass(name)檢查該類是否加載過(guò),加載過(guò)則直接返回該類的class對(duì)象 - 然后判斷是否有父類的
ClassLoader加載器,如果有則調(diào)用父類加載器加載該類,沒有則調(diào)用bootstrapClassLoader去加載該類 - 如何還有沒有找到,則調(diào)用自身的
findClass方法,所以當(dāng)我們自定義ClassLoader是需覆蓋此方法
/**
* Returns a class loaded by the bootstrap class loader;
* or return null if not found.
*/
private Class<?> findBootstrapClassOrNull(String name)
{
if (!checkName(name)) return null;
return findBootstrapClass(name);
}
該機(jī)制有效的保證了java核心類的正確加載,保護(hù)了java虛擬機(jī)的安全
